1. Field of the Invention
This invention relates generally to security systems used to secure the windows and doors of building structures against unauthorized entry. In particular, this invention relates generally to magnetically activated sensors used to detect unauthorized opening of windows or doors.
2. The Prior Art
It is known in the art of burglar alarm systems to use magnetically sensitive sensors to detect unauthorized entry. Standard magnetically sensitive sensors known in the art typically have two parts. The first part contains a magnet and is affixed to the moving portion of a window or door. The second part contains a magnetically sensitive switch and is affixed to the stationary part of the building, door jam or window sill. The stationary portion of the sensor is wired to an alarm system which is equipped to detect a change of state of the magnetically sensitive switch. Multiple windows or doors in a given room or area can be wired in a series loop where, when the area is secured, the loop is closed (conductive). If any of the sensors in a secured area are disturbed, the loop becomes open, activating the alarm. Typically, this type of system is used in a low risk level security system such as a typical home burglar alarm system.
Single switch magnetic sensors have the significant disadvantage of being easily defeated by an external magnetic field. A powerful magnetic field placed outside the window or door adjacent to the position of the sensor will hold the magnetically sensitive switch in a closed position even if the window or door is opened, thus defeating the ability of the sensor to detect movement of the window or door.
The sensors previously described are known as a Class 1, or low security level sensor. More sophisticated sensors are used in higher security systems. For example, a Class 2 sensor consists of a reed switch and bias magnet positioned in a receiver and an actuator magnet located in the transmitter. The reed switch is manufactured with the common leg touching the normally closed leg. Installation requires biasing the sensor by carefully positioning the bias magnet to move the common leg of the sensor out of contact with the normally closed wire and into contact with the normally open wire. The actuator magnet is capable of moving the switch back to the normally closed position. This switch is balanced and is held in the operational state only as long as the actuator magnet remains in a specific zone near the switch. If the actuator magnet is moved closer to the switch or further away, the switch becomes unbalanced, causing a change of state. Approaching a Class 2 switch with a magnet of the wrong polarity or of improper strength will cause the switch to change state and trigger the alarm. Class 2 switches offer a medium level of security and are used in commercial, industrial and prison security applications.
Switches of Classes 3-6 are known in the art, each providing a progressively higher level of security, complexity and expense. Class 2 switches, for example, are significantly more expensive than Class 1 switches and require extensive time and sophistication for proper installation and operation, making them impractical for inexpensive low security installations.
Briefly stated, the invention in a preferred form comprises a multi-combination magnetism transmitter and a multi-code combination sensor. The multi-code combination magnetism transmitter (hereinafter xe2x80x9ctransmitterxe2x80x9d) comprises a unit containing magnetic portions and non-magnetic portions arranged in a pattern. The multi-code combination sensor (hereinafter xe2x80x9creceiverxe2x80x9d) comprises multiple magnetically sensitive switches positioned to respond to magnetic fields in the transmitter. In practice, a magnetically sensitive normally open switch is positioned opposite the magnetic field of a magnetic portion in the transmitter. When correctly positioned, the magnetic field will close the magnetically sensitive, normally open switch. A magnetically sensitive normally closed switch is positioned opposite a non-magnetic portion of the transmitter. This normally closed switch will remain closed when it is positioned adjacent to the non-magnetic portion of the transmitter.
A pattern of magnetic and non-magnetic portions of the transmitter correspond to magnetically sensitive normally open and normally closed switches on the receiver, respectively. A matched transmitter and receiver form a sensor which can replace the single switch sensor and magnet of a standard Class I sensor while adding an additional level of security. Added security is provided by the magnetically sensitive normally closed switches. These normally closed switches will respond to an externally applied magnetic field used in an attempt to defeat the sensor.
The pattern of magnetic and non-magnetic portions in the sensor can be arranged in linear fashion where it will create a xe2x80x98binaryxe2x80x99 pattern of magnetic and non-magnetic portions. The magnetic, non-magnetic pattern may alternatively be more geometric, forming a grid of magnetic and non-magnetic portions. Whatever the form, the cost and installation complexity of the binary multi-code combination sensor is comparable to a standard Class 1 switch. The binary multi-code combination sensor is a direct replacement for Class 1 switches in existing low security alarm systems.
An object of the present invention is to provide a new and improved binary multi-code combination sensor which cost effectively provides an added level of security to standard burglar alarm systems.
Another object of the present invention is to provide a new and improved binary multi-code combination sensor equipped with multiple tamper detection switches.
These and other objects, features, and advantages of the invention will become readily apparent to those skilled in the art upon reading the description of the preferred embodiments, in conjunction with the accompanying drawings.